Add Vault support

[pacphi]

See https://learn.hashicorp.com/vault/kubernetes/sidecar. Overall this will result in a more secure footprint and allow for previously deployed applications to be patched.

There's some interesting work here because if we want to demonstrate an HA vault setup, we have some additional effort beyond a "dev mode" setup.

Take a look at these additional articles and videos for inspiration:

• https://blog.doit-intl.com/vault-high-availability-on-gke-68ef4fd7ca33
• https://medium.com/@amimahloof/automating-vault-secrets-consumption-in-kubernetes-via-mutation-webhook-eea1864d40b5
• https://www.hashicorp.com/resources/terraform-your-deployment-of-vault-on-kubernetes/

This work will support https://github.com/pacphi/tf4k8s/issues/10.